net80211: mitigation against A-MSDU design flaw

Mitigate A-MSDU injection attacks by detecting if the destination address
of a subframe equals an RFC1042 (i.e., LLC/SNAP) header, and if so
dropping the complete A-MSDU frame.  This mitigates known attacks,
although new (unknown) aggregation-based attacks may remain possible.

This defense works because in A-MSDU aggregation injection attacks, a
normal encrypted Wi-Fi frame is turned into an A-MSDU frame. This means
the first 6 bytes of the first A-MSDU subframe correspond to an RFC1042
header. In other words, the destination MAC address of the first A-MSDU
subframe contains the start of an RFC1042 header during an aggregation
attack. We can detect this and thereby prevent this specific attack.

This relates to section 7.2 in the 2021 Usenix "FragAttacks" (Fragment
and Forge: Breaking Wi-Fi Through Frame Aggregation and Fragmentation)
paper.

Submitted by:	Mathy Vanhoef (Mathy.Vanhoef kuleuven.be)
Security:	CVE-2020-24588
PR:		256119

(cherry picked from commit f024bdf1155f36d2d8c4caa533b66e4040c4c469)
(cherry picked from commit 41ca1d50a8657959df2009daa300dda56a090d5e)
(cherry picked from commit 76ee776f4d9f146f7a97ac9bab388c51a1c787c9)

Approved by:	so
Security:	FreeBSD-SA-22:02.wifi

6 files changed, 24 insertions, 7 deletions

diff --git a/sys/net80211/ieee80211_adhoc.c b/sys/net80211/ieee80211_adhoc.c
index ea1519b3381d..c97b99425caa 100644
--- a/sys/net80211/ieee80211_adhoc.c
+++ b/sys/net80211/ieee80211_adhoc.c
@@ -558,7 +558,7 @@ adhoc_input(struct ieee80211_node *ni, struct mbuf *m,
 		/*
 		 * Finally, strip the 802.11 header.
 		 */
-		m = ieee80211_decap(vap, m, hdrspace);
+		m = ieee80211_decap(vap, m, hdrspace, qos);
 		if (m == NULL) {
 			/* XXX mask bit to check for both */
 			/* don't count Null data frames as errors */
diff --git a/sys/net80211/ieee80211_hostap.c b/sys/net80211/ieee80211_hostap.c
index 6687276d0181..8c132332f86b 100644
--- a/sys/net80211/ieee80211_hostap.c
+++ b/sys/net80211/ieee80211_hostap.c
@@ -744,7 +744,7 @@ hostap_input(struct ieee80211_node *ni, struct mbuf *m,
 		/*
 		 * Finally, strip the 802.11 header.
 		 */
-		m = ieee80211_decap(vap, m, hdrspace);
+		m = ieee80211_decap(vap, m, hdrspace, qos);
 		if (m == NULL) {
 			/* XXX mask bit to check for both */
 			/* don't count Null data frames as errors */
diff --git a/sys/net80211/ieee80211_input.c b/sys/net80211/ieee80211_input.c
index 404eaf96716a..979e9aef9d12 100644
--- a/sys/net80211/ieee80211_input.c
+++ b/sys/net80211/ieee80211_input.c
@@ -294,7 +294,8 @@ ieee80211_deliver_data(struct ieee80211vap *vap,
 }
 
 struct mbuf *
-ieee80211_decap(struct ieee80211vap *vap, struct mbuf *m, int hdrlen)
+ieee80211_decap(struct ieee80211vap *vap, struct mbuf *m, int hdrlen,
+	uint8_t qos)
 {
 	struct ieee80211_qosframe_addr4 wh;
 	struct ether_header *eh;
@@ -316,7 +317,9 @@ ieee80211_decap(struct ieee80211vap *vap, struct mbuf *m, int hdrlen)
 	    llc->llc_snap.org_code[1] == 0 && llc->llc_snap.org_code[2] == 0 &&
 	    /* NB: preserve AppleTalk frames that have a native SNAP hdr */
 	    !(llc->llc_snap.ether_type == htons(ETHERTYPE_AARP) ||
-	      llc->llc_snap.ether_type == htons(ETHERTYPE_IPX))) {
+	      llc->llc_snap.ether_type == htons(ETHERTYPE_IPX)) &&
+	    /* Do not want to touch A-MSDU frames. */
+	    !(qos & IEEE80211_QOS_AMSDU)) {
 		m_adj(m, hdrlen + sizeof(struct llc) - sizeof(*eh));
 		llc = NULL;
 	} else {
@@ -364,6 +367,10 @@ ieee80211_decap1(struct mbuf *m, int *framelen)
 #define	FF_LLC_SIZE	(sizeof(struct ether_header) + sizeof(struct llc))
 	struct ether_header *eh;
 	struct llc *llc;
+	const uint8_t llc_hdr_mac[ETHER_ADDR_LEN] = {
+		/* MAC address matching the 802.2 LLC header */
+		LLC_SNAP_LSAP, LLC_SNAP_LSAP, LLC_UI, 0, 0, 0
+	};
 
 	/*
 	 * The frame has an 802.3 header followed by an 802.2
@@ -376,6 +383,15 @@ ieee80211_decap1(struct mbuf *m, int *framelen)
 	if (m->m_len < FF_LLC_SIZE && (m = m_pullup(m, FF_LLC_SIZE)) == NULL)
 		return NULL;
 	eh = mtod(m, struct ether_header *);	/* 802.3 header is first */
+
+	/*
+	 * Detect possible attack where a single 802.11 frame is processed
+	 * as an A-MSDU frame due to an adversary setting the A-MSDU present
+	 * bit in the 802.11 QoS header. [FragAttacks]
+	 */
+	if (memcmp(eh->ether_dhost, llc_hdr_mac, ETHER_ADDR_LEN) == 0)
+		return NULL;
+
 	llc = (struct llc *)&eh[1];		/* 802.2 header follows */
 	*framelen = ntohs(eh->ether_type)	/* encap'd frame size */
 		  + sizeof(struct ether_header) - sizeof(struct llc);
diff --git a/sys/net80211/ieee80211_input.h b/sys/net80211/ieee80211_input.h
index 810dcbde7978..5959319c76d7 100644
--- a/sys/net80211/ieee80211_input.h
+++ b/sys/net80211/ieee80211_input.h
@@ -311,7 +311,8 @@ void	ieee80211_deliver_data(struct ieee80211vap *,
 struct mbuf *ieee80211_defrag(struct ieee80211_node *,
 		struct mbuf *, int);
 struct mbuf *ieee80211_realign(struct ieee80211vap *, struct mbuf *, size_t);
-struct mbuf *ieee80211_decap(struct ieee80211vap *, struct mbuf *, int);
+struct mbuf *ieee80211_decap(struct ieee80211vap *, struct mbuf *, int,
+		uint8_t);
 struct mbuf *ieee80211_decap1(struct mbuf *, int *);
 int	ieee80211_setup_rates(struct ieee80211_node *ni,
 		const uint8_t *rates, const uint8_t *xrates, int flags);
diff --git a/sys/net80211/ieee80211_sta.c b/sys/net80211/ieee80211_sta.c
index 75f6c652c0fb..56372543a200 100644
--- a/sys/net80211/ieee80211_sta.c
+++ b/sys/net80211/ieee80211_sta.c
@@ -827,7 +827,7 @@ sta_input(struct ieee80211_node *ni, struct mbuf *m,
 		/*
 		 * Finally, strip the 802.11 header.
 		 */
-		m = ieee80211_decap(vap, m, hdrspace);
+		m = ieee80211_decap(vap, m, hdrspace, qos);
 		if (m == NULL) {
 			/* XXX mask bit to check for both */
 			/* don't count Null data frames as errors */
diff --git a/sys/net80211/ieee80211_wds.c b/sys/net80211/ieee80211_wds.c
index ea13572cf2d9..74cc764b856e 100644
--- a/sys/net80211/ieee80211_wds.c
+++ b/sys/net80211/ieee80211_wds.c
@@ -619,7 +619,7 @@ wds_input(struct ieee80211_node *ni, struct mbuf *m,
 		/*
 		 * Finally, strip the 802.11 header.
 		 */
-		m = ieee80211_decap(vap, m, hdrspace);
+		m = ieee80211_decap(vap, m, hdrspace, qos);
 		if (m == NULL) {
 			/* XXX mask bit to check for both */
 			/* don't count Null data frames as errors */